Wow, changes between 2.x and 4.x would be a very long list. It really depends on what features of the product you are leveraging.

Here are the big architectural changes:

* Share user interface, which I think is a big improvement over the Explorer interface in 2.x* CMIS APIs* Records Management (released in 3.2, and will be available in 4.x as 4.1)* Solr search, which should be more reliable and performent* IMAP interface* SharePoint protocol support* Tagging (I don't think this was available in 2.x, but I could be wrong)* Support for more transformations and more metadata extraction (EXIF, geotag, etc)* Replication between multiple Alfresco repositories* Better integration with WCM solutions using Web QuickStart* Social features* Automatic email notifications on activities and events* Publishing content to Twitter, Youtube, Facebook, etc* Doclib portlet